Modern utility-scale wind turbines have rotors comprising very long, slender blades. FIG. 1 shows a typical wind turbine blade 10, which tapers longitudinally from a relatively wide root end 12 towards a relatively narrow tip end 14. A longitudinal axis L of the blade 10 is also shown in FIG. 1. The root end 12 of the blade is circular in cross section. Outboard from the root, the blade has an aerofoil profile 16 in cross section.
The root of the blade is typically connected to a hub of the rotor via a pitch mechanism, which turns the blade about the longitudinal pitch axis L in order to vary the pitch of the blade. Varying the pitch of a blade varies its angle of attack with respect to the wind. This is used to control the energy capture of the blade, and hence to control the rotor speed so that it remains within operating limits as the wind speed changes. In low to moderate winds it is particularly important to control the pitch of the blades in order to maximise the energy capture of the blades and to maximise the productivity of the wind turbine.
The energy capture of a wind turbine blade generally increases moving from the root towards the tip. Hence, the inboard or root part 12 of the blade 10 tends to capture the least energy, whilst the outboard or tip part 14 of the blade tends to capture the most energy. Precise control over the pitch angle of the outboard part of the blade is therefore desirable in order to maximise the output of the wind turbine.
Modern wind turbine blades are typically 50-80 meters in length, and there is a constant drive to develop longer blades to capture more energy from the wind. These blades are generally made from composite materials such as glass-fibre reinforced plastic (GFRP). The blades are therefore relatively flexible and inevitably bend and twist to an extent during operation. The relatively narrow outboard part of the blade is particularly susceptible to twisting and bending.
Whilst the pitch mechanism allows precise control over the angle of the root of the blade, this does not necessarily reflect the angle of the tip of the blade, which is more susceptible to bending and twisting as mentioned above. The present invention provides a method and apparatus for measuring a twist angle of the blade tip accurately so that this information can be employed in control strategies. For example, accurate measurements of the twist angle can be employed in pitch control strategies allowing precise control over the angle of attack of the outboard part of the blade so that the energy capture of the blade can be maximised. The measurements may also be employed in blade load calculations and control strategies for protecting the blades from extreme loads.
The twist angle of the blade is defined herein as the angle between the chord line of the blade at the tip and a reference axis in a plane substantially perpendicular to the longitudinal axis L of the blade, as will now be described by way of example with reference to FIGS. 2a and 2b. The chord line is the straight line D connecting the leading edge 18 of the blade 10 to the trailing edge 20.
FIGS. 2a and 2b illustrate a cross-section of the tip of the wind turbine blade 10 in a plane substantially perpendicular to the longitudinal axis L and taken along the line A-A in FIG. 1. In FIG. 2a the blade 10 has a first twist angle, whilst in FIG. 2b the blade 10 has a second twist angle. The twist angle is marked θ in FIGS. 2a and 2b. The longitudinal axis L is substantially perpendicular to the plane of the page in FIGS. 2a and 2b. 
The L-y plane defines the plane of rotation of the rotor, and the x-axis is perpendicular to this plane. The direction of rotation of the rotor about a rotor axis is indicated by R in FIGS. 2a and 2b, which traces a circle in the L-y plane when the rotor is turned through an angle of 2π radians. The wind direction is indicated as W in FIGS. 2a and 2b. In FIGS. 2a and 2b the wind direction is illustrated as being perpendicular to the L-y plane, although in practice the direction of the wind relative to the L-y plane varies, and may be incident at different angles.
In FIG. 2a the blade tip twist angle 9 is defined as θ radians, that is, when the chord line D is parallel to the x-axis and therefore perpendicular to the L-y plane. FIG. 2b illustrates the blade tip turned through an angle θ with respect to the x-axis such that θ>0.
In the subsequent discussion of the invention, the above definition of the blade twist angle will be applied. In other words, the blade twist angle θ is defined with respect to an axis (the x-axis of FIGS. 2a and 2b) formed perpendicular to the plane of rotation (the L-y plane of FIGS. 2a and 2b) of the blade. It will be appreciated, however, that the twist angle may be defined relative to another arbitrary reference, and so this definition should not be accepted as unduly limiting to the scope of the present invention.
Modern wind turbines are very tall structures, and the blades are particularly susceptible to lightning strikes. Therefore, most wind turbine blades incorporate lightning protection systems for conducting the electrical energy from lightning strikes safely to ground. The present invention aims to avoid the use of metal parts or electrical components on wind turbine blades as these can attract lightning strikes in preference to the lightning receptors on the blade, which may cause damage to the blade. Present systems for measuring the degree of blade tip twisting are highly expensive and fragile. In contrast, the present system and method is both simple and inexpensive to implement, and is resistant to damage caused by the extreme weather conditions to which wind turbines are commonly subject.